This invention relates to electronic positioning devices, and more particularly to an improved apparatus and method for the positioning of transducing apparatus relative to a rotating storage medium.
In the past, there have been a wide variety of approaches taken to the positioning of transducing apparatus relative to a rotating disk storage medium. These approaches involve not only differing types of positioning schemes and associated apparatus, but also differing approaches to the control thereof. Positioning schemes have included linear-linear, rotary-rotary and rotary-linear techniques. Positioning apparatus typically employed have included stepper motors, voice coil actuators and non-commutated motors. Coupling between positioning apparatus and associated transducing apparatus has been both direct and indirect. While approaches to the control of positioning apparatus were originally analog in nature, more recent designs have been of a hybrid nature, employing both analog and digital techniques.
In the linear-linear positioning scheme, both the motion produced by a motor as well as the movement experienced by a transducing assembly relative to a rotating storage medium is rectilinear in nature. The motor and the transducing assembly may be coupled in either a direct or indirect manner. In the direct coupling arrangement, the motor is directly coupled to the transducing assembly in such a manner that there exist a one-for-one relationship between motion of the motor and motion of the transducing assembly. Such an arrangement has the advantage of producing an assembly which is quite rigid, having a high resonant frequency. However, the mechanical advantage of motion produced by the motor relative to that experienced by the transducing assembly is fixed, and consequently may not be optimized. As there is generally an optimal mechanical advantage in the design of such mechanical systems, a system of levers may be employed to maximize efficacy of the system. However, use of levers necessarily results in an indirectly coupled system. While an indirectly coupled system may provide for numerous advantages, including a method to maximize efficacy of the mechanical system as well as providing for a method to simplify position decoding through the use of encoding apparatus coupled to corresponding levers, such a system introduces a flexible member in the control train between the motor and the transducer assembly. Such flexible members, however, are undesirable due to the potential introduction of error in determining position of the transducer assembly.
In a rotary-rotary positioning scheme, both the motion produced by a motor as well as the movement experienced by a transducing assembly relative to a rotating storage medium is rotary in nature. The motor and the transducing assembly may be coupled in either a direct or indirect manner. In the direct coupling arrangement, the motor is directly coupled to the transducing assembly in such a manner that there exist a one-for-one relationship between motion of the motor and motion of the transducing assembly. Such an arrangement has the advantage of producing an assembly which is quite rigid having a high resonant frequency. However, the disadvantage of such a design is that the mechanical advantage of motion produced by the motor relative to that experienced by the transducing assembly is fixed, and as a result may not be optimized. In an alternate approach, a member is typically pivoted about a selected point with the transducing apparatus on one end, and the motor coupled to the other end. The distance between the pivot point and the two alternate ends of the member, i.e., the distance between the pivot point and the transducer assembly, and the distance between the pivot point and the point of coupling to the motor, may be selected in such a way to maximize the efficacy of the mechanical system.
In a rotary-linear positioning scheme, the motion produced by a motor is rotary in nature, and the movement experienced by a transducing assembly relative to a rotating storage medium is linear in nature. Due to the difference in the type of motion produced by the motor and the desired motion of the transducing assembly, the coupling between the two is necessarily indirect. Typical of such rotary-linear positioning schemes is the familiar band-drive system frequently employed in floppy disk based systems wherein a transducer assembly is moved in a radial fashion across a surface of a rotating disk by a flexible metal band which is coupled about a pulley affixed to a shaft of a motor. Rotation of the shaft of the motor consequently results in radial movement of the transducing assembly across the surface of the disk. By changing the diameter of the pulley, the efficacy of the mechanical system may be optimized.
Positioning apparatus employed with the foregoing positioning schemes have included stepper motors, voice coils and non-commutated motors. While stepper motors have found wide application as a positioning apparatus in various positioning schemes, positioning accuracy obtainable with a stepper motor is necessarily limited due to mechanical tolerances associated with the operation of stepper motors. Consequently, tracks on the storage media must be spaced far enough apart to allow for mechanical tolerances associated with the operation of the stepper motor actuator. While these tolerances may be acceptable in relatively inexpensive floppy disk based storage systems, such limitations are highly undesirable in hard disk based system where the primary emphasis is on storage capacity. Consequently the use of stepper motors is generally avoided as a positioning apparatus in a high capacity storage system due to the resulting limitations imposed on the storage capacity of the system.
Voice coils may alternately be employed to produce either rectilinear or rotational motion. However, notwithstanding the particular design thereof, the total displacement of the moving member of the voice coils used with positioning apparatus in disk systems in the past has been limited to the difference in length between that of the magnet and the length of the coil. Consequently, while voice coils do offer an advantage in speed in operation, the limited range of motion available therefrom is a limiting consideration.
A yet further alternate approach to the production of rotational motion in rotary-rotary and rotary-linear positioning schemes is found in the use of a D.C. motor. However, due to problems associated with the control of the commutation of such motors, commutation is generally not performed. Rather, the D.C. motor is typically operated over only a limited range of rotation; frequently less than 100 degrees. A number of disadvantages follow from such an approach. By limiting the amount of useable rotational displacement of the motor, larger diameter pulleys must be attached to the rotor of the motor to achieve increased amounts of linear displacements of the transducer apparatus assembly. This necessarily requires the use of larger amounts of space, which is often at a premium in high capacity storage systems. In further addition, by operating the motor over only a limited range of angular displacement, it is more difficult to achieve a match between the rotational inertia of the motor with the reflected inertia of the head assembly. As a result thereof, it is not possible to achieve the higher efficiencies with lower power dissipation which result from achieving an optimal match between the mass of the transducer apparatus assembly and that of the D.C. motor.
In addition to the foregoing, differing approaches have been taken to the control of the transducer positioning apparatus. In particular, analog techniques were first employed. However, numerous undesirable effects including variations in component tolerances resulting from environmental effects have resulted in a shift toward control circuity employing a hybrid of analog and digital techniques. Numerous hybrid approaches have been employed in positioning the transducer assembly, with varying degrees of digital control. Typical of such approaches is a hybrid approach described in U.S. Pat. No. 4,396,959 to Harrison et al. wherein transducer apparatus position information is processed using analog apparatus including analog peak detectors and sample and hold circuits, and wherein digital position control information is converted to an analog format and subsequently processed with analog summing circuits and loop compensation networks. Further hybrid signal processing techniques may be found in U.S. Pat. No. 4,419,701 to Harrison et al. wherein analog position information is processed with an analog peak detector circuit, and wherein position control information is converted from a digital format to an analog format for processing by analog summing circuits and loop compensation networks. While such approaches do offer varying degrees of improvements in the process of transducer apparatus position determination and control, the disadvantages present with the use of analog techniques are still present to varying degrees.
Apart from hybrid hardware designs employing both analog and digital techniques, numerous control techniques have been employed with respect to controlling the position of the transducer apparatus. Such techniques have exhibited varying degrees of accuracy in controlling the position of transducer apparatus. In one control algorithm more fully described in U.S. Pat. No. 4,419,701 to Harrison el al., an open loop mode of operation is employed until the transducer apparatus is positioned at a desired destination track on the storage media, thereafter changing to a closed loop mode of operation. In other approaches where the positioning apparatus may operate in a closed loop mode, position information is determined at somewhat irregular intervals until such time as the transducer apparatus is positioned at a desired destination track. Consequently, varying degrees of reliability have been achieved in the past in the control of the positioning of transducer apparatus relative to a rotating storage medium.
From the foregoing it is clear that notwithstanding the use of differing positioning schemes, i.e., linear-linear, rotary-rotary and rotary-linear, achievement of optimal matching between the prime mover, i.e., the device producing motion, and a payload, e.g. a transducer assembly, may result in the use of some system of levers, thereby precluding advantages available from a direct coupling arrangement. This disadvantage in part results from the somewhat limited range of motion available from prime movers. Consequently, there is a need to provide for an increased range of motion by prime movers. In addition to the foregoing, there is a further need not only for more reliable approaches to the control of transducer apparatus, but also for more fully exploiting the advantages available from digital techniques in the implementation of such approaches.